In one technique of carboxylating cellulose fiber, bleached cellulose wood pulp fiber is carboxylated in an aqueous slurry or suspension by addition of a primary oxidizer consisting of a cyclic nitroxide lacking any substitutable hydrogen atoms on either of the carbon atoms adjacent the nitroxide nitrogen. Nitroxides having both five- and six-membered rings have been found to be satisfactory. Both five- and six-membered rings may have either a methylene group or another heterocyclic atom (eg. nitrogen or oxygen) at the 4-position in the ring, and both rings may have substituent groups at this location.
A nitroxide catalyst added to the reaction medium is rapidly converted to the oxoammonium salt (primary oxidant) by a secondary oxidant such as chlorine dioxide. The oxoammonium ion then binds to a primary hydroxyl group or a hydrated C-6 aldehyde hydroxyl group of an anhydroglucose unit of cellulose on a cellulose fiber. In one proposed literature reaction mechanism, a hydroxide ion then abstracts a proton, thus breaking a carbon-hydrogen bond at the 6-position of the anhydroglucose unit undergoing oxidation. A molecule of the hydroxylamine form of the nitroxide is generated with the formation of each aldehyde group from a primary alcohol group or formation of each carboxyl group from a hydrated aldehyde group. The hydroxylamine form must then be converted to the nitroxide form by a single electron transfer to a chlorine dioxide molecule. The nitroxide form of the catalyst must then be converted (oxidized) to the oxoammonium salt form (active catalyst and primary oxidant) by a single electron transfer to chlorine dioxide. In each case, chlorine dioxide is reduced to chlorite ion.
The nitroxides may be formed in situ by oxidation of the respective hydroxylamines or amines. Oxoammonium salts of nitroxides are generated by oxidation of nitroxides in situ by the secondary oxidant. The oxoammonium salt of the nitroxide is the primary oxidant as well as the active catalyst for carboxylation of cellulose. Oxoammonium salts are generally unstable and have to be generated in situ from more stable nitroxide hydroxylamine or amine precursors. The nitroxide is converted to an oxoammonium salt, then undergoes reduction to a hydroxylamine during the cellulose carboxylation reactions. The oxoammonium salt is continuously regenerated by the presence of a secondary oxidant, such as chlorine dioxide. In general, since the nitroxide is not irreversibly consumed in the oxidation reaction, only a small amount of it is required. Rather, during the course of the reaction, it is the secondary oxidant which will deplete.
In cellulose carboxylation processes, elements of concern include the length of reaction time to provide the required carboxylation, and the amount of retention storage capacity in the catalytic carboxylation reactor required for that reaction time. A longer reaction time requires more retention storage capacity in the catalytic carboxylation reactor. Other elements of concern include reagent concentrations (e.g., of catalyst, secondary oxidant, and so forth), optimum reaction conditions (e.g., pH and temperature), and so forth.
If added carboxyl level of 2-12 milliequivalents (meq) per 100 g of oven dry (OD) cellulose fiber is desired, a single catalytic reaction with short reaction time and minimum retention storage capacity is generally sufficient. However, for many applications of carboxylated fibrous cellulose, higher levels of carboxylation, e.g. 20 meq, 40 meq, 100 meq, or even higher meq/100 g, are preferable.
It had been thought that only one addition of primary oxidant or catalyst at the beginning of the reaction would be sufficient to achieve a desired level of carboxylation because the regeneration of the primary oxidant would allow it to be reused.
However, as described in the inventor's co-pending U.S. patent application Ser. No. 13/604,331, the entire disclosure of which is incorporated herein by reference, it was discovered that it is more difficult to regenerate the active catalyst (i.e., the oxoammonium salt from the hydroxylamine precursor) as the carboxylation reaction continues, and that it is more difficult for the regenerated catalyst to find reactive sites on the cellulose as the carboxylation reaction continues.
Accordingly, in the aforementioned application, carboxylation methods in which both the primary oxidant and the secondary oxidant are supplied at intervals are disclosed. In particular, high levels of carboxylation of native (i.e. non-mercerized) cellulose fibers were achieved in a fast-flowing, continuous process using multiple catalytic carboxylation reactors with short reaction times (e.g., under 5 minutes) and therefore low retention storage volumes.
A continuing challenge, however, is to provide increased levels of carboxylation in a more cost-effective manner.